U.S. Pat. No. 4,970,561 to Mizuno, issued Nov. 13, 1990, shows a multicolor image forming apparatus in which a pair of toning stations are positioned alongside each other. Electrostatic images are formed on an image member and one or the other of the two stations is moved into position to tone each image. The apparatus includes a mechanism for moving one station into position and retracting the other simultaneously and vice versa. The drive mechanism for each station is engaged and disengaged according to which station is in position to tone the electrostatic image. The system is clutched by movement of the stations and does not require a separate clutch mechanism.
Recently, devices have been proposed in which toning stations do not have to be moved into and out of operative position with respect to an image member to control toning of an electrostatic image in a color apparatus. For example, U.S. Pat. No. 4,671,207 to T. K. Hilbert, Jun. 9, 1987; U.S. Pat. No. 4,690,096 to Hacknauer et al, issued Sep. 1, 1987; and U.S. Pat. No. 4,748,471 to Adkins, issued May 31, 1988, show a toning station which includes an applicator associated with an image member, which applicator is maintained in position to tone an electrostatic image. A transport device transports toner from a mixing device in a sump to the applicator when an image is being toned. The transport device includes a gating mechanism which terminates the flow of such developer when no image is being toned so that that image can be toned by another station without interference from this station. With this structure, the toning station can remain in a single location throughout use of the apparatus, greatly simplifying its construction. See also, U.S. Pat. Nos. 4,716,437 and 4,707,107.
Because some toner remains in the vicinity of the applicator when developer flow has been shut off, it is preferable in such structures to stop the rotation of the applicator when that station is not toning. This can be accomplished when using a single motor to drive two or more applicators, but it requires a separate clutching mechanism for each applicator. A more common solution is to use a separate motor for each applicator, which motor is turned on and off with the adjustment of the gating mechanism.